Method of representing a video image by means of a projector

ABSTRACT

The invention relates to a method of representing a video image based on a video signal by means of a projector which comprises an image display device and a high-pressure gas discharge lamp, which lamp is supplied with a square-wave alternating current (I 0 , I 10 ) on which a current pulse (P 1 , P 3 , P 10 , P 40 ) is superimposed before each phase reversal. According to the invention, the alternating current (I 0 , I 10 ) is superimposed with a second current pulse (P 2 , P 4 , P 20 , P 30 , P 50 , P 60 ) of the same polarity. A simple attunement of the alternating current frequency to the image frequency without image artefacts is made possible by the second pulses (P 2 , P 4 , P 20 , P 30 , P 50 , P 60 ).

The invention relates to a method of representing a video image based ona video signal by means of a projector which comprises an image displaydevice and a high-pressure gas discharge lamp, which lamp is suppliedwith a square-wave alternating current on which a current pulse issuperimposed before each phase reversal.

Projectors with such an image display device, referred to as a lightvalve and an array, are known from DE 694 24 858 T2 and EP 1154652. Theconstruction principle is explained in detail.

It is known from EP 11 52 645 A1 how a high-pressure gas discharge lampis operated with a square-wave alternating current for such a projector.The alternating current is superimposed with a current pulse of the samepolarity before the phase reversal at the end of each half cycle of thelamp current, i.e. before a polarity change. This avoids leaping of aluminous discharge arc and flickering of the image. This current changehas the result, however, that the lamp is now operated with analternating lamp current which comprises pulsatory components,increasing the luminous intensity in a corresponding pulsatory manner.The frequency of the alternating current is indicated as being anoperating frequency sequence of 45, 65, 90, and 130 Hz. It is alreadytaken into account with these values that the operating frequencyinfluences lamp life, and that lamp life will only be long if a lampfrequency of 30 to 200 Hz can be adjusted.

It is known from U.S. Pat. No. 6,278,244 to synchronize a square-wavealternating current comprising such a pulse for operating ahigh-pressure gas discharge lamp with a video signal.

Given a video frequency of 60 Hz and a lamp frequency of 90 Hz, a colorwheel can be rotated with such an angular speed that a video image basedon a video signal can be represented three times in a color sequence ofthe colors red, green, blue, and white, and the current pulse coincideswith the color white each time in the display of the image. Theoperating frequency of the lamp and the video frequency are mutually soattuned that the raised intensity of the light has no influence on thecolors of the image, and a neutral color impression is safeguarded. Adevice with these characteristics is manufactured under the name M3 bythe company of InFocus ASA of Fredrikstad, Norway, and is commerciallyavailable. Partial images, however, are still visible in the case ofmoving pictures. To avoid unpleasant artefacts caused by a sequentialcolor representation, it is desirable to increase a color changingfrequency. An increase in the frequency however, leads to a movement ofthe light pulse over individual color segments. The movement of thelight pulse leads to color shifts. A frequency increase of thealternating current leads to a shorter lamp life expectancy.

The invention accordingly has for its object to avoid color shifts andother image artefacts. In addition, the life expectancy of the lampshould be long.

This object is achieved by the characterizing features of claim 1.According to the invention, a second current pulse of the same polarityis superimposed on the alternating current. This additional pulse isgenerated within each half cycle and has no influence on lamp life. Thepulse can be generated by a simple modification in the control by thelamp driver. The second pulse renders possible a simple attunementbetween the lamp frequency and the image frequency without imageartefacts.

Advantageously, the current pulse occurs periodically. This achieves alight distribution corresponding to that in a lamp operation of doublefrequency. The commutation, however, still takes place at a lowfrequency, and lamp life remains the same because of the lowerfrequency.

Advantageously, the current pulse takes place aperiodically. The secondcurrent pulse thus takes place at different moments in time within thehalf cycle of the alternating current. The moment may be changed eitherin a random manner or in a given sequence. The advantage now is that thetime between mutually adjoining light intensifications is not laid down,but is variable. This implies that possible artefacts are not present infixed positions on the screen, but can be averaged out. This isinteresting for color wheels with spiraling color segments. Thesesegments generate horizontal color beams which run vertically downwardover the screen. All three colors are present on the screen at the sametime, but in different locations, see Dewald, Pa., Davis: “SequentialColor Recapture and Dynamic Filtering: A Method of Scrolling Color” inSID 01 Digest of Technical Papers, vol. XXXII, pp. 1076 to 1079, 2001,and Shimizu: “Scrolling Color LCOS for HDTV Rear Projection” in SID 01Digest of Technical Papers, vol. XXXII, pp. 1072 to 1075, 2001.

Advantageously, the second current pulse has the same contour as thecurrent pulse just before the phase reversal. A simple implementation ispossible when a pulse is used with the same contours as the pulse beforethe phase reversal.

Advantageously, the current pulse has a pulse duration that can bevaried. The second pulse can thus be attuned to the image displaydevice, in particular to a reflecting image display device. Overall, anexact pulse sequence can be optimized with respect to the addressschedule of the individual image display device.

Advantageously, the current pulse has an amplitude that can be varied.This renders possible a still more accurate adaptation to therequirements of the image display. An adjustment of moment, amplitude,and duration provides a possibility of setting parameters dynamically,i.e. parameters can be adapted in dependence on lamp age, image contentsto be displayed, or a chosen basic setting of the projection system. Toachieve a color temperature and color neutrality in a projector thathave been set, the first current pulses will occur in one of the colorsegments, for example red. The further current pulses will lie in othercolor segments and are adapted as regards their amplitude and duration.

Advantageously, the current pulse lies within a time period which isgiven by the final 80% of the total duration of a half cycle. In thismanner the lamp electrodes, and thus lamp life, remain unaffected.

Advantageously, the alternating current is synchronized with the videosignal. The pulses can thus be attuned to the colors of the colorsequence.

Advantageously, the current pulse occurs during a white segment. Colordistortions are avoided thereby, and a natural color representation canbe achieved.

Advantageously, the current pulse occurs during a color transition.Mixed colors are intensified thereby, together generating a white color,brightening up the image, and thus averaging each other out.

Advantageously, the frequency of the alternating current can be varied.A wide range of time intervals between consecutive lightintensifications can be generated thereby, and all periodic effects arelost. An application with additional pulses has the advantage ofgenerating pulse repetitions at a higher frequency which average outartefacts without low-frequency visible effects, while at the same timeretaining the lamp operation at an optimized low frequency.

Embodiments will be explained in more detail below for a betterunderstanding of the invention, with reference to the drawing, in which:

FIG. 1 is a time diagram of one cycle of an alternating lamp currentwith four pulses and a color sequence with the colors green, red, blue,and white, and

FIG. 2 is a time diagram of one cycle of an alternating lamp currentwith six pulses and a color sequence with the colors red, green, andblue.

FIG. 1 is a time diagram with a square-wave pulsatory alternatingcurrent I0 whose cycle has a duration of T0. The square-wave pulsatoryalternating current I0 is composed of a square-wave alternating currentIR and a pulsatory current IP. T0 is equal to 16 ms or a video frequencyof 60 Hz as used in NTSC video systems. A first pulse P1 ends at momentt1, i.e. after a half cycle before a phase reversal. The square-wavepulsatory alternating current I0 comprises a second pulse P2 which endsat moment t2, i.e. after a quarter of a cycle. The amplitude of thesquare-wave alternating current IR is denoted I2. The pulses P1 and P2are components of the pulsatory current IP, have pulse durations of tP1and tP2, an amplitude I3, and the same contour. When the alternatingcurrent IR and the pulsatory current IP are superimposed, having thesame polarity, the result is the pulsatory square-wave alternatingcurrent I0 whose highest amplitude is I4. A total of four pulses P1, P2,P3, and P4 occur in every cycle.

A color wheel with two segments for each of the colors green, red, blue,and white, i.e. a total of eight segments 1 to 8, rotates during analternating current cycle, equal colors being mutually diametricallyopposed.

A video image transmitted at 60 Hz is displayed four times, whichcorresponds to an image repetition rate of 4 or an image frequency of240 Hz, the repeated images also being referred to as sub-frames. Thelamp frequency can then be set for 60 Hz. Lamp life is long if a lampfrequency of 30 to 200 Hz, advantageously 60 to 120 Hz, in particular 90Hz can be adjusted. The pulses P1 to P4 occur during the white segments4 and 8.

FIG. 2 shows a square-wave alternating lamp current I10 with anamplitude I20 and with a total of six pulses P10, P20, P30, P40, P50,and P60 with an amplitude I30. The pulses P10, P20, P30, P40, P50, andP60 are spaced apart by one sixth of the lamp cycle T10 and end at themoments t10, t20, t30, t40, t50, and t60, i.e. after one sixth, onethird, one half, two thirds, five sixths, and one full period. Theduration of the pulses is the same each time and is denoted tP10, tP20,tP30, tP40, tP50, and tP60. The pulses P10, P20, P30, P40, P50, and P60have the same contours. The pulses P10 and P40 before a phase reversalare denoted anti-flicker pulses, the other pulses P20, P30, P50, and P60are denoted phantom pulses.

A color wheel uses the colors red, green, and blue without a whitesegment. Mixed colors still occur, however, upon each change to the nextcolor during a short period owing to a width of a light ray passingthrough the color wheel. These mixed colors, when summed together,generate white again. It is desirable, therefore, always to use a pulseP10, P20, P30, P40, P50, and P60 at each color change 10, 11, 12, 13,and 14. Given a 150 Hz sub-frame frequency and three color changes 10,11, and 12 per sub-frame, a pulse frequency of 450 Hz results, and therequired lamp frequency will be 225 Hz without phantom pulses. Fourfurther phantom pulses P20, P30, P50, and P60 are then an optimum ineach lamp cycle in addition to the two anti-flicker pulses P10 and P40,which results in an alternating current frequency of 75 Hz.

A distribution and an angle of the color segments is appliance-dependentand may vary. To improve a red rendering, it is possible to make a redsegment considerably larger than the other segments. This causes noproblems for a phantom pulse schedule, because the phantom pulses can befreely positioned. It is preferred for the lamp that the longest segmentappears after the phase reversal, which is also denoted commutation.

A Buck converter in U.S. Pat. No. 5,608,294 comprises means referencedIII which act as a control unit and which are in control of meansreferenced I and a commutator referenced II. The control unit III can beprogrammed such that the phantom pulses can be generated in addition tothe anti-flicker pulses.

1. A method of representing a video image based on a video signal bymeans of a projector which comprises an image display device and ahigh-pressure gas discharge lamp, which lamp is supplied with asquare-wave alternating current (I0, I10) on which a current pulse (P1,P3, P10, P40) is superimposed before each phase reversal, characterizedin that the alternating current (I0, I10) is superimposed with a secondcurrent pulse (P2, P4, P20, P30, P50, P60) of the same polarity.
 2. Amethod as claimed in claim 1, characterized in that the current pulse(P2, P4, P20, P30, P50, P60) occurs periodically.
 3. A method as claimedin claim 1, characterized in that the current pulse (P2, P4, P20, P30,P50, P60) occurs aperiodically.
 4. A method as claimed in claim 1,characterized in that the second current pulse (P2, P4, P20, P30, P50,P60) has the same contour as the current pulse (P1, P3, P10, P40) beforethe phase reversal.
 5. A method as claimed in claim 1, characterized inthat the current pulse (P2, P4, P20, P30, P50, P60) has a pulse duration(tP2, tP20, tP30) that can be varied.
 6. A method as claimed in claim 1,characterized in that the current pulse (P2, P4, P20, P30, P50, P60) hasan amplitude that can be varied.
 7. A method as claimed in claim 1,characterized in that the current pulse (P2, P4, P20, P30, P50, P60)lies within a time period given by the last 80% of the total duration ofa half cycle.
 8. A method as claimed in claim 1, characterized in thatthe alternating current (I0, I10) is synchronized with the video signal.9. A method as claimed in claim 1, characterized in that the currentpulse (P1, P2, P3, P4, P1, P20, P30, P40, P50, P60) takes place during awhite segment (4, 8).
 10. A method as claimed in claim 1, characterizedin that the current pulse (P1, P2, P3, P4, P10, P20, P30, P40, P50, P60)occurs during a transition (10, 11, 12, 13, 14) from one color toanother.
 11. A method as claimed in claim 1, characterized in that thefrequency of the alternating current (I0, I10) can be varied.
 12. Aprojector for a method as claimed in claim
 1. 13. A projector forrepresenting a video image based on a video signal, comprising an imagedisplay device and a high-pressure gas discharge lamp, which lamp issupplied with a square-wave alternating current (I0, I10) on which acurrent pulse (P1, P3, P10, P40) is superimposed before each phasereversal, characterized in that the alternating current (I0, I10) issuperimposed with a second current pulse (P2, P4, P20, P30, PS0, P60) ofthe same polarity.